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Research

RF Arrays

Radiofrequency (RF) arrays play a crucial role in MRI. In all modern MRI systems, arrays of RF coils are used to detect the signals emanating from the spins in the human body. Larger number of elements in such receive arrays can increase the SNR and allow for higher acceleration of the acquisition.

While clinical MRI systems at 1.5T and 3T use birdcage volume coils for transmission and arrays for reception, at ultra-high field (UHF) MRI, RF arrays are not only used for reception, but also for transmission. This is to gain more degrees of freedom when shaping the transmit field, as volume coils do not produce a homogenous excitation field at UHF. Similar to beam forming in modern radar technology, the elements of a transmit array in MRI are driven with individual amplitudes and phases and even different pulse shapes.

These arrays can be characterized be a multitude of criteria. They should have a high transmit efficiency to provide a strong excitation field with the transmit power provided by the system, as well as a high SAR efficiency to allow for a strong excitation field without damaging tissue due to excessive heating. Furthermore the elements of the array need to be well decoupled and their respective transmit profiles need to be distinct from one another to provide a high degree of freedom for shaping the transmit field.

Left: 32-channel transmit array for large field of view imaging used at the DKFZ's 7T MRI. The array is mounted on the bore liner. Right: Inside view of one half of the array. 16 transmit elements in 3 rows are visible.
© dkfz.de

While transmit RF arrays at UHF are almost exclusively local arrays placed directly on the body, our group aims at developing RF arrays that are integrated in the MRI system behind the inner cladding of the bore, just like the transmit volume coils of lower field MRIs [1].

Our current and future work in this area includes:

Finding transmit antennas best suited for integrated arrays
Developing very thin arrays to fit between boreliner and gradient coil of the most modern MRI systems
Developing meta material solutions for improved coil performance

References:

Orzada, S., et al. (2019). "A 32-channel parallel transmit system add-on for 7T MRI." PLoS One 14(9): e0222452.

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